The present invention relates to an apparatus for counting clock slips or drifts on digital transmission links such as satellite links, cable links or radio links.
The digital flow rates presently encountered on such links are 2,8,34 and up to 140 Mbit/s as a function of the link and sometimes more, or different values in the case of US standards, namely 1.5,6,45 Mbit/s, etc.
The informations transmitted for said links are time multiplexed and are known as MIC links.
It is pointed out that a MIC link for a digital primary block, i.e. a 2 Mbit/s link, is in the form of frames T subdivided into time intervals IT, each frame having a time interval ITO reserved for frame synchronization, time intervals reserved for data (voice channels in this case) and sometimes a time interval IT16 reserved for signalling. The number of IT is 32.
An 8 Mbit/s frame is constituted by sectors, the sector I comprising a frame locking word FL. A 34 Mbit/s frame is also constituted by four sectors, the first starting with a frame locking word FL. A 140 Mbit/s frame is constituted by six sectors, the first starting with a frame locking word FL.
Only the structure of the 2 Mbit/s frame has the special feature of one frame locking word for two IT's. Thus, said word is constituted by seven bits contained in the ITO of a frame Tn and a second bit of the ITO of the frame Tn+1 (consecutive).
It is also pointed out that a clock slip or drift is the result of a fast or slow variation (drift) of the clock frequency of the digital primary block for a 2 Mbit/s link (of the secondary digital block for an 8 Mbit/s link, etc.). This drift leads to a jump or a doubling of bits causing a frame locking loss. It is also pointed out that a frame locking loss is not only due to clock slips, but also to other transmission errors.
It is known to carry out a clock slip count for 64 kbit/s and 2 Mbit/s links and several methods exist.
Reference can e.g. be made to a first method, which is the analog method consisting of using a time reference. This reference is obtained by means of a precision clock with which measurement takes place of the drift of the clock of the primary digital block (block corresponding to the 2 Mbit/s link). Although this method is very accurate it is expensive as regards costs and equipment. Reference can be made to the article "Frequency and Time" by Rohde & Schwartz--info 001 104 E-1 for further details.
Reference is also made to a second, digital method. It consists of inserting a pseudorandom sequence into the frame, which makes it necessary to reserve one IT for said sequence. A reception analysis of said IT is carried out to establish whether or not there has been a slip.
This method suffers from the disadvantage of requiring complicated means and also of not being transparent on transmission. Therefore, it leads to the loss of one IT for useful information transmission, said IT being reserved for the pseudorandom sequence. Further details are provided in the CCITT G822 recommendation.
Another method is known which involves testing consisting of carrying out a life data analysis (LDA) and inter alia makes it possible to perform clock slip measurements. This procedure consists of using two test equipments. A first equipment receives the transmitted signal and analyses it for transmitting a check list by a data link to the second equipment, which receives said list and the informations of the transmission line. This second equipment also produces a check list and compares it with that which it has received.
This method suffers from the disadvantage of requiring two equipments and a data link.
The present invention aims at obviating these disadvantages.
The present invention more specifically relates to a process for counting clock slips in a digital transmission link constituted by frames (or sectors) indicatable by a frame locking word, characterized in that it involves the following stages;
initiating a first detection of the frame locking words of the transmission link with comparison with a reference frame locking word,
initiating a second detection of the frame locking words, time lagged compared with the first detection, as soon as an error has been detected by the latter,
determination of the frame locking loss and resumption on the basis of predetermined criteria,
counting clock slips when there has been a determination of a frame locking loss at the end of the first detection and a frame locking resumption at the end of the second detection.
According to a second feature of the process, the initiation of a second detection consists of comparing the content of the first IT of the frames succeeding one another with a reference frame locking word, detection taking place when identity exists.
According to a third feature of the process, the initiation of a second detection consists of seeking in the frame succeeding that for which there has been a detection of an error at the end of the first detection, the frame locking word by successive comparison of each content of IT with the reference frame locking word, detection taking place when identity exists.
According to a fourth feature of the process, the determination of the frame locking loss takes place when, at the end of the first detection, a given number n of errors has been obtained and a given number p of errors has been obtained at the end of the second detection, p being below n, a transmission error other than a clock slip being detected.
According to a fifth feature of the process, the determination of frame locking resumption takes place as soon as a frame locking word is detected during the second detection.
According to another feature of the process, in the case where the digital signal is a 2 Mbit/s signal, the frame locking word being constituted by the content of the ITO of an even frame Tn and the second bit of the ITO of the odd frame Tn+1, the determination of the frame locking loss taking place when three consecutive ITO of even frames are received with error and/or three consecutive ITO of odd frames received with errors and frame locking resumption takes place when at a given instant t1 there is the presence of a correct ITO of one frame Tn and then at the instant t2 the second bit of the ITO of the frame Tn+1 is correct and that at an instant t3, the ITO of the frame Tn+2 is correct.
The present invention also relates to an apparatus for counting the clock slips of a digital transmission link constituted by frames (or sectors) indicatable by a frame locking word, characterized in that it comprises:
first detection means for the frame locking words of the link,
second detection means for frame locking words activated by the first means when said first means have detected an error on a frame locking word,
means for the determination of the frame locking loss or resumption by first and second detection means,
means for counting the clock slips activated by the frame locking loss and resumption determination means when, on the one hand, there has been a frame locking loss by the first detection means and, on the other hand, when there has been a resumption by the second detection means.
According to another feature, the apparatus also comprises means (1) for compensating line attenuations of the monitored digital signal.
According to another feature, the apparatus also comprises means (2) for transforming the coded signal received into a binary signal.
According to another feature, the first and second means (30,40) respectively constitute a first and a second frame locking seeking modules each having a general time base circuit (3,4).
According to another feature, the frame locking loss and resumption determination means comprise the first frame locking module (30), a bit counter (8), two synchronization modules (6,5), a frame locking word presence detection module (70) and a frame counting and storage module (60).